1. Technical Field
The present invention relates generally to a printing apparatus, and more particularly to an improved shifting mechanism for a thermal head relative to a platen roller of a printing apparatus.
2. Background Art
Thermal printers are well known in the art wherein a thermal head is pressed against a platen roller by the tensile force of a coil spring to print an image on a sheet of paper. The printing station of such a printer is provided with a thermal head and a platen roller. During printing operations, two positional relationships between the thermal head and the platen roller are provided. One is a first operational mode in which the thermal head is separated, or shifted away from the outer peripheral surface of the platen roller when printing is not being executed. The other is a second operational mode in which the thermal head is pressed against the platen roller to print an image on a sheet of paper.
In the first operational mode, a sheet of paper is inserted between the thermal head and the platen roller. In the next second operational mode, the platen roller rotates upon contact of the thermal head to begin printing. Usually, a spring and a linkage or cam is utilized for providing the two operational modes. For example, a head shifting mechanism which includes a coil spring and a cam is well known in the art. In this mechanism, the thermal head is supported by a retaining plate which swings about a shaft according to rotation of the cam.
When printing starts, a predetermined angular displacement of the cam by a drive motor causes the thermal head to be shifted down to the outer peripheral surface of the platen roller under pressure of the coil spring. After printing, a further angular displacement of the cam by a given angle causes the thermal head to be shifted away from the platen roller while further compressing the coil spring. Thus, this shifting of the thermal head requires large torque from the drive motor to work against the compression force of the coil spring.
With this arrangement, the thermal head is controlled to be automatically shifted up and down. However, in the shifting mechanism, if the thermal head is left in contact with the platen roller under pressure, a compression set occurs on the platen roller due to the pressure of the thermal head. On the other hand, if the thermal head is left separated from the platen roller, the sheet of paper tends to shift due to vibrations caused by operation of the drive motor or so forth. Further, shifting of the thermal head after printing allows the printed paper sheet to become dislodged from its correct position, inducing error in cutting the printed out paper sheet at a predetermined position on the paper.